The Garnet Of Chicheng-Chongli Hongqiyingzi Complex In Northern Hebei: Genesis And Significance

The genesis of the garnet within the garnet-biotite plagiogneiss and theretrograded eclogite in Chicheng is different. The protolith of retrograded eclogitemaybe the tholeiite, which both possess the nature of the island arc and themid-ocean ridge, and underwent High temperature and high pressure metamorphismduring the process of subduction. The protolith of garnet-biotite plagiogneiss may bethe politic (or pelitic-bearing) sandstone and formed in the tectonic environmentsimilar to continental island arc, and underwent regional amphibolite faciesmetamorphism. Take the SHRIMP U-Pb dating of the zircon in the garnet-biotiteplagiogneiss and the retrograded eclogite in Chicheng and the North China regionaltectonic evolution in consideration: Because of the exhumation of the retrogradedeclogite and its intrusion into the garnet-biotite plagiogneiss, the latter underwentthe granulite facies metamorphism, and both underwent regional amphibolite faciesmetamorphism caused by the orogenesis in the late period，With the closure of thePaleo-Asian Ocean, overthrust to Chicheng areas, followed by the Indo-Yanshanianorogeny transformation.The component band features of the garnet within garnet-biotite plagiogneiss andgarnet-plagioclase amphibolite in Chongli are basic unanimously: Both showtemperature reduction and pressure increase, that is to say, experienced a similarmetamorphic evolution, but the genesis is different. The protolith of garnet-biotiteplagiogneiss in Chongli may be the politic (or pelitic-bearing) sandstone and formedin the tectonic environment, and the protolith of the garnet-plagioclase amphibolite inChongli may be the calcium-alkaline basic volcanic rock series. The temperature andpressure of the garnet-plagioclase amphibolite is T=627℃, P=0.9422GPa, which is calculated by the hornblende-plagioclase thermobarometer. The temperature andpressure of the garnet-biotite plagiogneiss is T=695℃~701℃, P=0.9658GPa~0.9720GPa. According to the component band features of the garnet and thecomposition of the garnet of the different metamorphic facies, the garnet-biotiteplagiogneiss and the garnet-plagioclase amphibolite in Chongli maybe experienced asimilar metamorphic evolution.The garnet within the garnet-biotite plagiogneiss in Chicheng-Chongli bothbelong to almandine-grossular-pyrope series. Both the core and the rim of the garnetwithin the garnet-biotite plagiogneiss in Chongli, the pyrope is lower than the ones inChicheng. The garnet within the garnet-biotite plagiogneiss in Chicheng showtemperature reduction and pressure decrease, and the changes of the temperature andpressure is synchronized. The garnet within the garnet-biotite plagiogneiss in Chonglishow temperature reduction and pressure increase, and the changes of the temperatureand pressure is non-synchronous. The differences of the garnet within thegarnet-biotite plagiogneiss in the two areas, indicate that the hongqiyingzi complex inChicheng-Chongli experienced different tectonic evolution, the hongqiyingzi complexshould be disintegrated.